Extended linear power amplifier systems are often used in radio frequency communication systems, such as multicarrier cellular systems or other wireless communication systems. Extended linear power amplifier systems typically include a number of cascaded linear power amplifiers that output signals that are combined, as known in the art, into a single high power signal before transmission. The number of linear power amplifiers may be any suitable number and may be selectively shut down to adjust power consumption output by a transmission unit, for example, a cellular base station.
Such linear power amplifiers may employ feedforward error correction circuits for a group of cascaded linear power amplifiers, such as dual loop feedforward error correction circuits. Accordingly, improved linearity may be achieved with such linear power amplifiers. Accordingly, the main amplifiers of a linear power amplifier may be coupled to one, two or more feedforward error correction loops. The main amplifier circuit may include a plurality of cascaded power amplifiers that are coupled in a parallel fashion. The main amplifier circuits are typically larger and draw more current than the associated feed loops.
Although improved linearity may be provided by, for example, two feedforward loops coupled to a main amplifier circuit, each loop adds some amount of loss. One example of the operation of a feedforward amplifier may be found, for example, in U.S. Pat. No. 5,831,479, having inventor Leffel et al., issued Nov. 3, 1998, owned by instant assignee and incorporated herein by reference. Although conventional linear power amplifier systems may selectively turn off linear power amplifier stages, additional power savings may be desirable. In addition, a linear power amplifier having feedforward error correction may achieve its linearity to a suitable level without the requirement of one or more feedforward loops.
Accordingly, the linear power amplifiers may operate at different operating points depending upon received signal. It would be desirable to provide a linear power amplifier that is able to switch between different operating modes to offer power savings advantages at different operating points. Different operating modes include for example, enabling or disabling one or more feedforward loops; converting feedforward error amplifier circuits to temporary main amplifier circuits; or enabling or disabling all or parts of the main amplifier circuit.
Consequently, there exists a need for a linear power amplifier that provides improved efficiency in linear power amplifiers employing feedforward error correction circuits.